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A140252 as a simple table

na(n)
00
11
21
37
47
531
631
7127
8127
9511
10511
112047
122047
138191
148191
1532767
1632767
17131071
18131071
19524287
20524287
212097151
222097151
238388607
248388607
2533554431
2633554431
27134217727
28134217727
29536870911
30536870911

[0,1,1,7,7,31,31,127,127,511,511,2047,2047,8191,
 8191,32767,32767,131071,131071,524287,524287,
 2097151,2097151,8388607,8388607,33554431,33554431,
 134217727,134217727,536870911,536870911]